1. Field of the Invention
The present invention relates to an abrasion amount estimation device and an abrasion amount estimation method for check valve of an injection molding machine, and especially relates to an abrasion amount estimation device and an abrasion amount estimation method for check valve of an in-line type injection molding machine.
2. Description of the Related Art
In an in-line type injection molding machine, a check valve provided on an end of a screw makes relatively-advancing and relatively-retreating movement during injection and metering so as to prevent resin from flowing in a rear direction of the screw during injection and allow the resin to flow from the rear direction to the forward direction during metering. Here, if the check valve does not retreat during injection or timing of movement of the check valve is not stable even when the check valve retreats, a problem occurs in which mass of a molded article does not reach a predetermined value due to a backflow phenomenon of resin toward the screw backward direction or variation in mass of molded articles is generated.
Such problem in movement of a check valve is attributed mainly to change of a dimension of the check valve from the initial dimension caused by abrasion thereof resulted from prolonged molding. Accordingly, in order to stably produce high-quality injection molded articles in a long period, it is necessary to monitor whether or not a check valve is abraded, and to promptly exchange the check valve for a new check valve when abrasion is detected.
As a method for detecting an abrasion amount of a check valve, a method in which a screw is regularly taken out from an injection cylinder so as to directly measure the dimension is known. However, in this method, the measurement operation has to be performed in a state where production is temporarily suspended, thereby degrading productivity disadvantageously. Therefore, as a method for indirectly detecting an abrasion amount without taking out a screw from an injection cylinder, a method for detecting the backflow phenomenon of resin toward the screw backward is known. For example, Japanese Patent Application Laid-Open No. 01-168421 and Japanese Patent Application Laid-Open No. 2008-302527 disclose a method in which a load torque which is applied in a rotation direction of a screw is measured so as to detect the backflow phenomenon of resin. Further, Japanese Patent Application Laid-Open No. 01-281912 and Japanese Patent Application Laid-Open No. 2009-096045 disclose a method in which an advancing speed of a screw during packing process is measured so as to detect the backflow phenomenon of resin. Furthermore, Japanese Patent Application Laid-Open No. 2008-302528 discloses a method for estimating an abrasion amount of a check valve based on a screw position detected when the check valve is closed.
The methods for detecting abrasion of a check valve disclosed in the above-mentioned prior art techniques have a merit that abrasion of a screw can be indirectly detected even during production through detection of the backflow phenomenon of resin. However, operation conditions of an injection molding machine during production vary depending on molded articles or kinds of resin, and a load torque and an advancing speed of a screw which are indices of the above-mentioned backflow phenomenon also vary depending on operation conditions of the injection molding machine. Thus, it is hard to estimate an abrasion amount of a check valve with high accuracy.